support --backend as a pytest flag to limit to one backend for testing
pass posargs via tox so --backend can be used for tox envs
don't mutate _ALL_BACKENDS
better name for the variable
modify backend selection to allow multiple backends via comma delimiter
docs for explicit backend selection and document name attribute of backend
revert fixture decorator for now, switch to append. no more globals
update docs for name attribute
fix docs
expand tox backend example
Merge pull request #468 from reaperhulk/backend-flag-pytest

Add backend flag for pytest runs